A U.S. worker inspects a section of the U.S.-Mexico border wall at Sunland Park, U.S. opposite the Mexican border city of Ciudad Juarez, Mexico(C) Jose Luis Gonzalez / Reuters / Reuters

March 7, 2017, 7:27 PM GMT / Updated March 7, 2017, 7:28 PM GMT

By Catalina Gonella

The majority of immigrants settling in the U.S. without authorization are first coming to the country legally, raising questions about the effectiveness of President Donald Trump's plan to build a wall on the U.S.-Mexican border.

Crossing the border is not the way "the large majority of persons now become undocumented," the Center for Migration Studies (CMS) said in a recent report. Two-thirds of those who joined the undocumented population did so by entering with a valid visa and then overstaying their period of admission, the center repored.

Overstays have exceeded those entering illegally every year since 2007, and there have been half a million more overstays than illegal entries since 2007.

The study by the CMS divides the 2014 population into two groups: those who enter with a valid temporary visa, but subsequently “overstay” and establish residence in the U.S. without authorization and those who “enter without inspection” (EWI) through the southern border without proper immigration documents.

According to the report, in 2014, 42 percent of all undocumented persons in the U.S. were “overstays.”

Of those who arrived or joined the undocumented population in 2014, 66 percent were overstays.

This trend is expected to continue.

The CMS has previously reported on the dramatic decline in the U.S. undocumented population between 2008 and 2014.

“Despite the claims of an ever-rising, out-of-control U.S. undocumented population,” said Donald Kerwin, CMS’s executive directors said in the 2016 report, “the number of undocumented has fallen each year since 2008.”

The 2016 report also found that a growing percentage of people crossing the border into the U.S. are migrants from Central America who are presenting themselves to U.S. authorities and seeking political asylum.

Since inauguration, the Trump administration has made the construction of a “contiguous, physical” and “impassable” wall along the 2,000-mile U.S.-Mexican border a significant part of its immigration enforcement strategy and directed its construction in an immigration executive order.

Trump’s plan to build a wall had already been widely criticized, not only for its estimated cost of $21 billion, but for necessity, given the enforcement tools already in place, amongst other reasons.